A much more complicated, generic, typesafe lens implementation for Typescript

lens.ts

function lens<K extends string>(key: K) {
  return <const>{
    get: function<R extends any>(record: R): K extends keyof R ? R[K] : unknown {
      const k: string = key
      return record[k]
    },
    set: function<R extends any>(record: R, value: R[K]) {
      return record.hasOwnProperty(key) ?
          {
            ...record,
            [key]: value
          } as const
        :
          record
    }
  }
}

type Lens = ReturnType<typeof lens>

const nameLens = lens('name')
const fakeLens = lens('fake')

const data = {
  name: 'Bob',
  age: 23
}

const x: string = nameLens.get(data)
// Error on getting property that doesn't exist
const y: string = fakeLens.get(data)

const z = nameLens.set(data, 'Roberto')
// Error on incorrect value type
const zz = nameLens.set(data, 123)

function compose(a: Lens, b: Lens) {
  return <const>{
    get: function<R extends any>(record: R) {
      return b.get(a.get(record))
    }
  }
}

const nested = {
  payload: {
    id: '123'
  }
}

const payloadLens = lens('payload')
const idLens = lens('id')
const nestedLens = compose(payloadLens, idLens)

// Annoyingly, id has lost its type here... 
const id = nestedLens.get(nested)
console.log(id)